1. Field of the Invention
The present invention generally relates to a filter device, and more particularly, to a filter device that has piezoelectric thin-film resonators housed in a package.
2. Description of the Related Art
It has been widely known that a ladder-type surface acoustic wave (SAW) filter element having single-terminal pair SAW resonators (hereinafter referred to simply as “resonators”) arranged in series arms and parallel arms can exhibit filter characteristics in a broader band area, with inductance components being connected in series to the resonators arranged in series arms or parallel arms. It has also been known that, with those inductance components, the filter characteristics can be highly restrained in the vicinity of pass bands.
Japanese Unexamined Patent Publication No. 5-183380 (hereinafter referred to as “Prior Art 1”), for example, discloses a structure having the inductance components (L101 through L105 shown in FIG. 1) formed with bonding wires that connect the electrode terminals of the surface acoustic wave filter element and the electrode terminals of the package. FIG. 1 illustrates the circuit structure of the ladder-type SAW filter device 100 disclosed by Prior Art 1.
To obtain a smaller device, it is also effective to connect the electrode terminals of a surface acoustic wave filter element to the electrode terminals of a package with bumps, with the surface acoustic wave filter element facing down. Japanese Unexamined Patent Publication No. 2002-141771 (hereinafter referred to as “Prior Art 2”), for example, discloses this type of structure. FIGS. 2 through 4 illustrate the ladder-type SAW filter device 200 disclosed by Prior Art 2. FIG. 2 is a section view of the ladder-type SAW filter device 200, taken along the transmission direction of surface acoustic wave (SAW). FIG. 3 is a top view (surface B) of a ladder-type SAW filter element 201 that is mounted on the ladder-type SAW filter device 200. FIG. 4 shows a die-attach face A of a package 202 that houses the ladder-type SAW filter element 201.
As shown in FIG. 2, the ladder-type SAW filter device 200 has the ladder-type SAW filter element 201 face-down bonded to the die-attach face A of the package 202 with bumps 203. To connect an inductance component in series to each resonator 214 shown in FIG. 3, microstrip lines 220 are formed on the die-attach face A of the package 202 in Prior Art 2, as shown in FIG. 4.
In the surface acoustic wave filter element 201 of Prior Art 2, however, wires 210 between the surface acoustic wave resonators 214 and wires 210 between the surface acoustic wave resonators 214 and bump connecting pads 213 might be long, as shown in FIG. 3. In that case, the wires 210 function as inductances, adversely affecting the filter characteristics of the surface acoustic wave filter device 200.
More specifically, the wires 210 are normally formed by performing a vapor-phase epitaxy process on a thin film in the same stage as forming comb-like electrodes 211 and reflection electrodes 212 of the surface acoustic wave resonators 214. Therefore, the film thicknesses of the wires 210 are 0.05 μm to 0.5 μm. Such wires 210 formed with thin films have large resistance losses, and function as inductances with poor Q characteristics. In the disclosure of Prior Art 2, the filter characteristics of the surface acoustic wave filter device 200 deteriorate due to the addition of inductances with poor Q characteristics.
In the disclosure of Prior Art 2, bump connecting pads 221 formed on the die-attach face A of the package 202 occupy quite large areas to secure a predetermined precision in the production process, as shown in FIG. 4. More specifically, a margin is allowed for the positioning accuracy in the bonding process using the bumps 203, and the sizes of the bump connecting pads 221 are increased accordingly. When the ladder-type SAW filter element 201 is face-down bonded to the package 202 using the bumps 203, the conductor pattern (the comb-like electrodes 211 and the reflection electrodes 212) formed on the surface B of the ladder-type SAW filter element 201 faces the bump connecting pads 221 on the package 202, resulting in a problem of large stray capacitance that adversely affects the filter characteristics of the surface acoustic wave filter device 200.
Further, the ladder-type SAW filter element 201 disclosed by Prior Art 2 has a substrate made of LiTaO3 or LiNbO3 having a large dielectric constant of 30 to 50. When the ladder-type SAW filter element 201 is face-down bonded to the package 202, the stray capacitance generated in the conductor pattern on the die-attach face A of the package 202 becomes large, and adversely affects the filter characteristics of the surface acoustic wave filter device 200.